For the purpose of process monitoring during printing, it is usual to provide printed control strips with colored test patterns outside the subject on sheets or webs to be printed. These control strips, whose longitudinal direction is transverse with respect to the transport direction of the printing material, contain a set of measurement areas which repeat periodically in the longitudinal direction, on each of which a specific characteristic variable that characterizes the printing quality can be measured. An image of at least part of the control strip is acquired and evaluated during the movement of the printed product to be examined in the press. As an alternative to a control strip provided specifically for the purpose, in principle a portion of the printed useful area of the printing material can also be recorded and evaluated.
DE 195 38 811 C2 describes an apparatus of the generic type in which a digital image of part of a control strip is recorded by an electronic camera as the control strip moves through the area of observation of the camera. Flash lamps, which are either gas discharge lamps or incandescent lamps, are provided for the purpose of broadband illumination of the control strip during its presence in the area of observation of the camera. In order to permit color-selective evaluation of the image, a color camera is used. By means of a color-selective beam splitter, the different spectral components of the light falling into the camera are distributed to three different image sensors, one each for the red, green and blue spectral ranges. Color cameras of this type have a complicated structure and are accordingly costly. In addition, they are inferior to black-and-white cameras in terms of sensitivity. Furthermore, each color camera has a predefined spectral sensitivity characteristic, which cannot be changed from the outside.
In the case of densitometers, DE 196 17 009 C2 discloses the concept of illuminating a measurement point sequentially in time with light-emitting diodes (LEDs) of different colors and receiving the reflected light by using a single optoelectronics sensor which is not color-selective, specifically a silicon photodiode. The signals recorded one after another during different illumination phases yield information about the spectral composition of the reflected light and therefore also about the color of the printing ink present at the measurement point. The densitometer described in the aforementioned reference is, however, designed for only an approximately point-like measurement, since the light from three light-emitting diodes of different colors is aimed at a single measurement point, either directly as a result of their arrangement or indirectly by means of optical waveguides. Furthermore, this densitometer is also provided only for measuring on a stationary printed product after its removal from a press, i.e., for offline operation, in which the duration of the individual illumination phases can be selected freely in order to utilize optimally the dynamic range of the sensor employed.